* gc.c: add two GC tuning environment variables.
  RUBY_GC_MALLOC_LIMIT_MAX and RUBY_GC_MALLOC_LIMIT_GROWTH_FACTOR.
  See r43067 for details.
* gc.c (rb_gc_set_params): refactoring. And change verbose notation.
  Mostly duplicated functions get_envparam_int/double is not cool.
  Please rewrite it.
* test/ruby/test_gc.rb: fix a test for this change.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43072 b2dd03c8-39d4-4d8f-98ff-823fe69b080e